It is well known that silage has to be treated and stored under special conditions in order, as far as possible, to maintain its nutritional content for winter feeding. During storage, a number of chemical changes take place, and it can be important to know a number of nutritional parameters of the silage, for example to facilitate determination of when the silage is ready to be used. One of the most important parameters is the reducing sugar content. Also, generally in the production of food and drink for human and animal consumption and with other biological liquids, there are many instances when it would be valuable to be able to assess readily the reducing sugar content.
Reverting to silage, there is already known an analysing system which is able to determine various nutritional parameters of a liquor obtained by squeezing a silage sample, including the free sugar content. However, this system is relatively expensive and complex, and depends on wet chemistry. Such a system is described in GB 2202637A.
In the field of biosensors, there is known from EP 0170375A a capillary-fill device (CFD). In a common form, this comprises two plates separated by a gap sufficiently small that liquid can be drawn into it by capillary action. The internal faces of the plates carry printed electrodes, and may also carry a reagent layer deposited, e.g. by screen printing. This reagent layer is selected with due regard to the liquid to be drawn in, to carry out a particular test. EP 0255291A discloses a CFD including an oxidase enzyme specific for a particular sugar to be detected, e.g. glucose oxidase for determination of the concentration of glucose in a liquid, for example a blood sample. This device is thus specific to a particular sugar so that only one particular sugar can be tested at a time.
It is also known to determine the sugars content of a solution by making use of the reducing properties of sugars, for example involving reduction of ferricyanide ion to ferrocyanide ion. The resulting ferrocyanide ion is then detected by a convenient method, for example spectrophotometrically, or possibly electrochemically, although spectrophotometric methods have generally been preferred (e.g. by measuring the decrease in yellow colour of a solution, when the ferrocyanide ion system is used). Traditionally this has been done at an alkaline pH, for example at pH 9-12, in a hot solution in the presence of a catalyst (e.g. Ni.sup.2+, Co.sup.2+). Such a technique usually takes around thirty minutes to provide a result to the user.
The quantitative determination of sugars by making use of their reduction properties has also been done at acidic pH's; however, in this case the products detected are produced by a different pathway, in which the sugar hydrolyses. The total reducing sugars determination is made by measuring the resulting concentration of the reduced form of the reactant compound, for example the ferrocyanide ion concentration, usually by spectrophotometric methods, or occasionally potentiometrically.
The determination of the reducing sugars content of a solution using conventional spectophotometric techniques has required the facilities and trained personnel of a laboratory. Also, the methods and techniques traditionally used for the determination of the total sugars content of solutions have often been time consuming and laborious.
Another known technique of sugars determination is pulsed amperometric detection, e.g. as described in "Triple-Pulse Amperometric detection of Carbohydrates after Chromatographic separation", Hughes and Johnson, Analytica Chimica Acta, 149 (1983), 1-10. This technique involves oxidising the sugar in a sample directly at a working electrode. This technique is most generally applicable for the determination of single sugars (such as for example are analyzed for when eluting a sample from a chromatographic column), and is only applicable to relatively small amounts of sugar (of the order of nanomoles). This technique also has the disadvantage that it requires the application of several different potentials to the test device, thereby requiring that the method be carried out in several different steps. The method is also only suitable to be carried out in an analytical laboratory, and requires the use of expensive equipment and skilled operators.
Not only for testing silage liquors, but for testing various aqueous liquids related to food and drink production, it would be highly advantageous to provide a simple method and device for measuring on site the sugar content of an aqueous liquid, not dependent on the presence in the liquid of any one particular sugar, and more especially with the aim of measuring the total reducing sugar content of the liquid.